This battle has shown the value of an unelected Lords

The defence of civil liberties is often a defence of the liberties of very unpopular minorities

The fraught parliamentary passage of the Prevention of Terrorism bill gives rise to two fundamental but inter-related constitutional questions. The first is how civil liberties are to be protected in a country without a codified constitution; the second is what ought to be the relationship between the Commons and the Lords.

The fraught parliamentary passage of the Prevention of Terrorism bill gives rise to two fundamental but inter-related constitutional questions. The first is how civil liberties are to be protected in a country without a codified constitution; the second is what ought to be the relationship between the Commons and the Lords.

In Britain, despite the Human Rights Act, we remain primarily dependent upon Parliament for the protection of our rights. Is this sufficient during periods of moral panic? The late Lord Scarman thought not. "When times are normal," he declared in his Hamlyn lectures in 1974, "and fear is not stalking the land, English law sturdily protects the freedom of the individual and respects human personality. But when times are abnormally alive with fear and prejudice, the common law is at a disadvantage; it cannot resist the will, however frightened and prejudiced it may be, of Parliament."

Until the 21st century, internment without trial had been introduced only in wartime and to meet the special problems of terrorism in Northern Ireland. The Anti-Terrorism Act of 2001, debated for only 16 hours in the Commons, broke that restraint, at least as far as non-British citizens were concerned.

The new Bill has been introduced after parts of that were ruled incompatible with human rights legislation and extends the executive's power to restrain without trial through control orders and house arrest to British citizens as well. This Bill too raises the fundamental issue of whether the deprivation of liberty at the wish of the executive can be justified in peacetime. Hence the determined civil liberties opposition in the House of Lords.

The House of Lords, an unelected chamber, must of course be constitutionally subordinate to the Commons. That subordination is recognised in the 1949 Parliament Act, which provides that the Commons can override the Lords by passing the same bill in two successive sessions. In practice, however, disputes between the two houses rarely come to this, since the Lords generally recognise the primacy of the elected chamber and give way gracefully. As it seemed to, although not so graciously, last night.

That still leaves the fundamental question of civil liberties and how do we defend them in this country - an issue far from being resolved by the final compromise of later review.

The upper house has been strengthened by the 1999 House of Lords Act, removing all but 92 of the hereditary peers. No peer, therefore, owes his or her position solely to hereditary right. Every peer has been specifically chosen, either by being nominated - the life peers - or by election - the 92 hereditaries. The new House is more legitimate than the old. Therefore, it can be more assertive, as it has just showed.

The conflict between the two houses and its confused end exposes in stark fashion the continuing inability of the British system of government to protect basic rights. The courts, admittedly, can issue a declaration of incompatibility when they are unable to interpret legislation in accordance with the Human Rights Act. But they still rely on government and Parliament to alter the law; they cannot themselves strike down legislation, as the US Supreme Court, for example, can. Therefore, the only institutional defence against threats to civil liberties is the unelected chamber.

There has been much talk in recent years of reforming the House of Lords so that part or the whole of it comes to be directly elected. It is worth asking, however, whether a directly elected upper house would be as good a friend to civil liberties as the reformed House of Lords is proving to be. It is worth remembering that the defence of civil liberties is often a defence of the liberties of very unpopular minorities - illegal immigrants, suspected paedophiles and suspected terrorists. These are not the kinds of people likely to do well in the electoral marketplace.

The House of Lords, it was once said, represents no one but itself and enjoys the full confidence of its constituents. Perhaps, however, a second chamber which is not responsible to a democratic electorate is precisely what Britain needs if civil liberties are to be properly defended - that is, until we follow the path of the vast majority of democracies by embracing a codified constitution and a genuine bill of rights able to trump the sovereignty of Parliament.

The writer is professor of government at Oxford University